I debated where to put this message. I put it here over the small-engine forum as the crux of my question is the history of ZDDP as an oil additive.

One of my hobbies is the collection, restoration, and operation of vintage engines. Most of these are small air-cooled engines, most often made by Briggs and Stratton. In one of the old-engine forums, a gentleman was looking for information on oil for a Briggs engine where the manual called out for "nothing heavier than SAE 20."

His question: was the SAE 20 in the mid-50's the same as SAE 20 today?

I answered back noting that viscosity measuring technology had not changed, but that oil formulation had changed. Using my spreadsheet of oil viscosities collected from various product data sheets, I noted that the viscosity of SAE 20W-20 oils (I couldn't find data on straight SAE 20) when hot were comparable to modern 5W-20 oils.

I also noted that before he run out and buy 5W-20 for this engine, he should consider that these old engines are flat-tappet engines, and that a modern PMCO oil may not have enough ZDDP in it. (I also noted that for an engine that isn't run under load, say at engine shows, it might not matter.)

I received a reply back noting that:

Quote:

These engines were designed and in production before the use of ZDDP in engine oil. So they are fine to use on modern oil. Even many flat tappet auto engines are just fine, depends on the design. ZDDP is really only needed in some of the poorly designed auto engines from the 60's through 80's.

My questions are thus:

1) When did ZDDP start being used as an oil additive?2) Anyone care to comment on if or not an old flat-tappet engine, such as a 1935 to 1955 or so Briggs and Stratton, will do okay on PMCO?

PersonallyI don't know when ZDDP came into use, but as far as using it in old B+S engines, I have been using whatever is cheapest #30 HDEO in my 1974 6 hp on a small rider since new. One thing you have to realize with small engines, is the valve spring pressure. Probably the cam isn't as hard as an automotive cam, but then the springs are so weak you can literally compress them with your fingers, so don't think you need to much if any zddp to offset that much pressure. ( P.S. I have been a professional small engine mechanic for 45 years. )Then at the same time, I do want it (zddp) in my 2 classic cars, just to be safe.

ZDDP became commercially available after WWII. In 1951 the API SB classification established some anti-scuff properties that would call for the use of ZDDP in order to meet them.

So, an engine that was designed before 1951 never took into account the use of any additives, but this doesn't mean that using oil with ZDDP in that engine would have an adverse effect on it. Up until today, using a higher classification oil than the engine was designed for is OK, but using an earlier classification in a newer engine is not recommended.

I don't have the API specs for those classifications, but take as an example the ACEA oil sequences, the A5/B5 oil limits the sulfated ash to 1.6 % and the C1 oil limits the ash to 0.5%; but both oils need to pass the same valve train scuffing test with the same wear limit of 15 microns. But how the C1 oil, with less metallic additives than A5/B5, achieves the same performance? Because of the use of more expensive non-metallic additives.

An old engine doesn't need to use C1 oil, since it does not have a catalytic converter to protect, but the use that oil will offer the same protection to the valve train as the A5/B5 oil.

Regarding antique engines, any modern PCMO should do fine. You may want to use a thicker grade though, such as 0W-40 or 10W-40. In addition to thicker oil film, xW-40 has more ZDDP as well. xW-30 might work just fine, too.

Here is all about ZDDP. I can't post the whole ZDDP review article because it's copyrighted:

Zinc dithiophosphates (ZDDPs) are arguably themost successful lubricant additives ever invented. Theywere introduced over 60 years ago, have been in continuoususe ever since and are still being employed inpractically all current engine oils. This longevity is allthe more striking since strenuous efforts have beenmade by additive companies over the last 10 years toreplace them, but in vain. It has so far proved impossibleto identify any reasonably cost-effective compoundhaving comparable antiwear performance to ZDDPsin engine oils.

As well as being remarkable in their performance,ZDDPs have also been astonishingly successful in theirability to inspire research. The last half century hasseen an extraordinary number of published researchpapers describing investigations of how these additivesbehave in their triple role as antioxidants, corrosioninhibitors and antiwear agents.

...

It is not yet clear whether the limits of phosphorusand sulphur in engine oils will be reduced further infuture, leading perhaps to the eventual disappearanceof ZDDP. Recently, attention has started to focus onthe possibility of replacing a blanket limit on the levelof phosphorus and sulphur in engine oils to a measurethat better reflects the tendency of these elements tovolatilise and thus reach the after-treatment catalyst[15]. This may eventually lead to a limit on P- andS-containing additive volatility or to a test which monitorsthese species in the exhaust and thus permitsimaginative new formulations based on low volatilityadditives [13]. Whatever the future however, there isno doubt that the slow pace of reduction of phosphoruslevels in engine oil specifications over the last5 years reflects the remarkable effectiveness of ZDDPas an antiwear additive, and the great difficulty thatadditive companies have had in finding a replacementwith comparable performance.

...

4.8. Antiwear properties of ZDDP

When considering the mechanisms by which ZDDPprevents wear, it is important to note that ZDDP isboth an antiwear and a mild EP additive, i.e. it bothreduces wear and also inhibits the onset of scuffing.This was recognised in the 1960s, when the influenceof metal type and alkyl group structure on wear andEP behaviour were measured and compared [35,36].Antiwear effectiveness was found to correlate inverselywith thermal stability of the ZDDP but this trend wasless clear-cut with respect to EP effectiveness [35]. Severalstudies have suggested that the antiwear behaviourof ZDDP results from its ability to form a phosphatefilm while its EP response results from its ability toform iron sulphide [53,130]. This is consistent withother antiwear and EP additives; sulphur-free phosphorusadditives are often effective antiwear but generallyineffective EP additives, while organic sulphides,although possessing some wear-reducing capability aregenerally regarded as EP additives [131,132]. Similarly,studies have shown that in mild rubbing conditions thesurface film present is mainly a thick phosphate filmbut that in severe, heavily loaded/high sliding speedconditions a much thinner film with high sulphur contentis formed [53]. Thus we need to distinguishbetween the ‘‘mild-wear’’ and ‘‘severe wear’’ action ofZDDP, the latter being essentially an EP response.This EP aspect will not be discussed in detail in thisreview except to note that studies of thermal degradationof ZDDP have shown that most of the sulphurpresent in these molecules is converted to oil-solubleorganic sulphides and disulphides and that these arewell-known EP additives.

From the literature, three main ways that ZDDPacts as an antiwear agent have been proposed; (i) byforming a mechanically protective film; (ii) by removingcorrosive peroxides or peroxy-radicals; (iii) by‘‘digesting’’ hard and thus abrasive iron oxide particles.Each of these is discussed briefly below.

The most generally accepted view of ZDDP antiwearaction is that the reaction film acts simply as amechanically protective barrier [133]. This preventsdirect contact and thus adhesion between metal ormetal oxide surfaces and may also operate as a cushion,reduces the stresses experienced by the asperitypeaks of the metal substrate. The relative importanceof these two effects has not been determined. With thistype of antiwear action, once a ZDDP film forms,practically all that wear that subsequently occurs ispresumed to be that of the ZDDP film itself. (In fact,ZDDP tribofilms appear to be very resistant to wearand several studies have shown that once formed theyH. Spikes/The history and mechanisms of ZDDP 483are only very slowly worn away even when the ZDDPcontainingoil is replaced by a base oil [89,103]). Inthis case, in mild wear conditions, the only loss of substratemay be from iron oxide which has reacted toform a phosphate film.

The second proposed mechanism of the antiwearaction of ZDDP is that it reacts with peroxides in thelubricant; thereby preventing these from corrosivelywearing the metal surfaces present [69,70]. This mechanismwas convincingly demonstrated by both Habeeband Rounds in the 1980s and no subsequent work haschallenged it.

The third suggestion is more controversial. Martinand colleagues have proposed that iron oxide particles,that would cause abrasive wear, embed in theZDDP antiwear film and are ‘‘digested’’ to formrelatively soft iron phosphate, thus negating theirharmful pro-wear effect [62,64,129]. This modelappears to have been inferred from identification ofiron phosphate in wear particles and in the rubbingtrack rather than any direct evidence of iron particledigestion. It does seem likely that iron oxide fromthe metal substrate diffuses into the ZDDP reactionfilm to replace some of the zinc cations with iron onesand form iron phosphates, and recently SIMS depthprofiling has been used to show that there is a muchlower iron oxide concentration beneath the ZDDPtribofilm that on the surrounding metal surfaces[96]. What is lacking as yet is direct evidence thatharmful iron wear particles are removed by a digestionprocess.

One interesting aspect of ZDDP wear performancethat has arisen very recently is that ZDDPs appear tostrongly promote micropitting wear. Micropittingresults from localised plastic deformation due to thesurface loadings resulting from rolling/sliding asperitycontact and it has been shown that ZDDP, because itvery rapidly forms a protective film, prevents or postponeseffective running-in of rough surfaces. This leadsto high asperity stresses being maintained and consequentmicropitting [134]. What is not yet clear is theextent to which this is an undesirable feature of all antiwearadditives or of ZDDPs in particular.